Blow Molding & Plastics Discipline

Since 1999

Blow Molding Help and Associates

BLOW MOLDING TECHNOLOGY

 

More and more people are becoming involved in Blow Molding.  As this happens, inevitably some need a quick primer as to what kind of machine does what kind of job.  What follows is an explanation of how the process works and how the particular machine types differ from one another.  Generally the accumulator head machines are used to make industrial parts and the continuous extrusion machines are used to make bottles.  The reciprocating screw machines are generally cross over that can make containers as well as industrial parts (mostly dairy bottles).  The IBM machines are generally make health and beauty aid products as well as bottles.  The re-heat and blow machines (both one-step and two-step) generally make bottles out of PET (Polyether ester terathyalade).  Most of the PET products are bi-axially oriented and are used to contain carbonated beverages.

 

The advantages of the blow molding process are the creation of a hollow part in which the molds are usually made out of aluminum (although not in the injection blow process).  Aluminum is of course much softer than steel, has a much higher heat transfer value and is far easier and quicker to machine.  Thus the tooling cost for the blow molding process is far less than in the injection molding process.

THE BLOW MOLDING PROCESS

 

The machine is pre-set for the parameters of the particular resin to be molded. The plastic material is placed into the hopper and fed into the mouth of the extruder. The extruder has a heated barrel with a screw inside to convey the pellets down the barrel while heating them.  The liquefied plastic is then forced around the mandrel which forms it into a tube which is then pushed out of the head by a plunger.  The orifice of the head has tooling that is programmed in the original step to form the particular wall thickness of the part by increasing or decreasing the annulus. The parison (name for the tube) is now hanging in mid air and the mold halves close on it.  An air source either by a blow pin or a needle blow then blows the part to the shape of the mold cavity. The mold is then re-opened and the part is removed from the machine and secondarily trimmed or finished as necessary.

 

 

TYPES OF MACHINES THAT ARE COMMONLY USED IN BLOW MOLDING

 

Continuous extrusion machine (generally bottle machines).

 

Continuous extrusion means that the extruder is constantly producing a parison (tube) out of its head.  The mold halves then grab the parison and then transfer it to a blow station where the air molds the part to the configuration of the mold.

 

Accumulator head blow molding machine.

 

These are generally used for industrial parts in which the melted material is accumulated in the head and then pushed out from the head by cylinders.

 

Reciprocating screw blow molding machine.

 

These machines are a semi injection machine and a semi accumulator head machine.  They first melt the plastic and collecting the shot in front of the screw and then push it out over the mandrel creating the parison and then forms the part in the same way as type 1 and type 2.

 

Injection blow machine or IBM’s.

 

These are machines that are a cross between an injection molding machine and a blow molding machine.  They first squirt liquefied plastic material into a closed mold (steel) forming a pre-form.  The machine then opens its clamp and indexes the pre-form on a mandrel to a blowing station where the air is applied to form the part (usually a bottle) into the shape desired.  The machine then indexes a second time to the ejection station.

 

Two step re-heat and blow machine or RHB.

 

In this process an injection molded pre-form is unscrambled and placed into a serpentine belt system and re-heated.  (The re-heating is programmed by quartz heaters to allow the exact form to more easily be blown when it reaches the mold cavities.)  When it reaches the mold cavities a rod pushes the parison thereby lengthening it and simultaneously blow air is supplied through the mouth of the container thereby stretching the pre-form in two directions at the same time.  This produces a bi-axially oriented product which is capable of providing a co2 barrier thus making a typical pop bottle.

 

One-step blow molding machine.

 

The process is similar to the above IBM process ( #4) in that a pre-form is molded in the first stage mold halves, indexed to the second stage and  stretched and blown at the same time as in step #5.

 

The particular materials that are generally blow molded are:

 

PET (poly ester tetrat

HDPE (high density polyethylene)

HWPE (high molecular weight polyethylene)

Nylon

ABS (acrylonitrile butadiene styrene)

PVC (polyvinyl chloride)

Polycarbonate

Engineered materials such as zenoi, ppo and other common injection molded materials

Silicone gum rubber

 

As you can see from the above list most commonly injection molded and extruded materials can be blow molded.  The particular materials are chosen for their physical properties, cost and environmental utilization properties.

 

The sizing of the particular machine is based on the weight of the part plus the flash of the part (if flashed) and the particular molecular weight of the resin.  For example if one were going to run a 5 gallon gas can (the red ones in your garage).  One would have to first know that it weighs approximately 1500 grams, cycles in about 48 seconds, is molded out of HDPE and generally is molded on a single cavity or double cavity machine.  The key point in the above example is that the 1500 gram finished weight of the product has a flashed weight of about 2000 to 2100 grams. Thus a five pound machine (accumulator head) is necessary to mold the product.  It can also be molded in a reciprocating machine as well as in a continuous extrusion machine.  In the case of the continuous extrusion machine it becomes a bit more difficult to determine what size is necessary.  In all cases the platens must be large enough to accommodate the mold. With continuous extrusion one must understand that the out-put is continuous and therefore one must know the cycle time of the product in order to determine the pounds per hour that the extruder must produce.  In the case of the red gas can the machine must be capable of 350 to 400 pounds per hour to keep up with the cycle time.

 

All of the above is a very cursory explanation of the sizing of the machines versus the nature of the product.  An additional factor to come into play is the capability of the head to accommodate the tooling which makes the parison the correct diameter.  Each machine is capable of a tooling size range.  In the case of the red gas can we would need a tooling size of approximately 7 inches in diameter.  The way that this is calculated is to first use the circumference of the circle.

 

C=pd

The circumference is then divided by two thereby giving us lay flat.

 

Lay Flat = C=pd

                      2

The lay flat is really the parison squeezed flat which is what happens when the mold halves close on the tube.

Parison Diameter X Pie-3.14 ÷ 2 = Lay flat

In other words the diameter of the tooling determines the diameter of the tube and thus the size of the product that can be made by that head tooling.  Each machine has a particular maximum and minimum tooling diameter thereby allowing the machine to be size to make the particular product.  

 

As one might well understand a few paragraphs do an injustice to the technology.  This little bit of information is provided on-line to help the user in sizing and selecting the machinery necessary for their job.  More information and help in sizing any machinery can be provided by using the contact at the end of this web site.  As time permits more and more data will be put on this web site.  We invite your comments and help in fulfilling this task.  BJ-JMG